1. Field of the Invention
The present invention relates to a hybrid integrated circuit (hybrid IC), in which a microchip and a group of other electronic devices are located on a ceramic circuit board. The microchip and the group are respectively covered and protected with a different type of potting material.
2. Description of the Related Art
As shown in FIG. 1, in a proposed hybrid IC, a microchip 3 is located on a surface 1a of a ceramic circuit board 1. The microchip 3 is die-bonded to the surface 1a with conductive adhesive 4. The microchip 3 and bonding pads of wiring layers (not shown) are wire-bonded with bonding wires 5. The microchip 3, the bonding wires 5, and the bonding pads are covered and protected by a first potting material 7 such as epoxy resin.
The wiring layers, except for the bonding pads, are covered and protected by an insulating protective layer (not shown). Therefore, the protective layer is in contact with the periphery of the potting material 7 on the board 1 at an end of the protective layer, which surrounds the device 3. The adhesion between the protective layer and the potting material 7 is relatively poor, so the potting material 7 tends to peel after heat cycles during use or after a soldering step in the manufacturing process of the hybrid IC.
As a solution to the peeling issue, in JP-A-11-40710, an elastic thermosetting resin is placed between the protective layer and the potting material 7 to reduce the stress generated at the boundary between the protective layer and the potting material 7. The stress is caused by the difference in the thermal expansion coefficient between the protective layer and the potting material 7. However, the method of the publication requires a dedicated material and apparatus, which are relatively expensive. Therefore, the production cost of the hybrid IC is increased.
In the proposed hybrid IC, as shown in FIG. 1, other electronic devices 9 such as condensers are fixed to the surface 1a of the circuit board 1 with the conductive adhesive 4. Other electronic devices 9 are covered by a second potting material 8 such as silicone rubber. As shown in FIG. 1, the first potting material 7 tends to be in contact with one of the electronic devices 9 because the distances between the device 3 and other electronic devices 9 are reduced due to the requirement of high packing density. If the first potting material 7 is in contact with one of the electronic devices 9, the first potting material 7 and the conductive adhesive 4 tend to crack after heat cycles during use due to the stress at the boundary between the first potting material 7 and the electronic device 9. The stress is caused by the difference in the thermal expansion coefficient between the first potting material 7 and the electronic device 9.